@lbejaran
You are so right that you have earned the right to have the garage space. Don't back down, we are all right behind you - shaking our fists! Just kidding. Too bad little sis is coming to the garage with you, it sounds like you have earned your privacy. Also, he needs to learn that he needs to earn what he wants in life. ON a side note: Did he ever finish the coop he was building?
Nivia and I are accustomed to sharing a room (a much smaller room, to be honest

We get along very well, so it's easier for us to share a space. I'm glad someone is shaking their fists at him, haha! I do that but no one else seems to agree.

As for the coop, it is not finished. It is just sitting on the patio, taking up space. I'm gonna probably take over as soon as he gives me his plans. I commented that I had never built a coop before and he was rude and said, "Yeah, I can tell from your coop". He's arrogant sometimes. I snapped at him after that and he apologized.
Even if he's coming in to visit your sister, he still needs to knock first. What if one of you is nekkid?
Well, we are in the habit of locking doors when we change (we are all fairly private and learned early on to do so), but being half naked has been a problem. I go to sleep in boxers and a tshirt, so I don't like for people to come into the room while I sleep. It takes a couple hours for the room to get nice and coop, so I don't use a blanket at first.
The issue has been that he looks for her and either I don't hear him knocking or he just opens the door.
Hence the need for a new door handle and a couple spoken rules so he knows to stay away.